1. Field of the Invention
The present invention relates to a fundus camera for photographing a fundus of an eye to be examined.
2. Description of Related Art
For a fundus camera, a 35-mm film is historically used as a basic image pickup medium, and a display frame (a mask or a field diaphragm) is fixedly arranged to limit a photographing range to a surface of the film which is an image forming surface. Also, for a recent fundus camera capable of electronic photography (image-pickup), a display frame is used, which is similar to that arranged when the 35-mm film is used.
Besides, in the fundus camera, focus adjustment to a fundus surface is performed by moving a focusing lens in order to correct a refractive error of an eye to be examined. However, when the focusing lens is moved, a photographing magnification is changed, and an image circle of a fundus image expands outside the display frame or shrinks inside it.
FIGS. 4A to 4C are views showing a state of the fundus image obtained when the focusing lens is moved in order to correct the refractive error. Reference numeral 100 is an objective lens, 101 is a focusing lens which moves along a photographing optical axis in an optical-axis direction, 102 is an image forming lens, 103 is a display frame being fixedly arranged, 104 is a relay lens, and 105 is a CCD camera which is an image pickup medium. If the fundus image when the focus adjustment is made with the eye to be examined under the conditions of emmetropia (0D(diopter)) as shown in FIG. 4A is taken as a reference, the fundus image when the focus adjustment is made with the eye to be examined under the conditions of myopia (−D) as shown in FIG. 4B forms such a shape where an image circle 106 thereof is expanded outside an display frame image 103a. On the other hand, the fundus image when the focus adjustment is made with the eye to be examined under the conditions of hypermetropia (+D) as shown in FIG. 4C forms such a shape where an image circle 107 thereof is shrunk inside the display frame image 103a (the blurred image circle 107 appears). Incidentally, S is a position of the lens 101 when the focus adjustment is made with the eye to be examined under the conditions of emmetropia (a standard position).
As mentioned above, the display frame fixedly arranged determines an actual photographing range of the fundus, however, the display frame may exactly be in consistent with a field angle only in the case where the eye to be examined is under the conditions of emmetropia. To cope with this problem, there is no other solution but to take a measure such that the blurred image circle does not appear even if the eye to be examined is not under the conditions of severe hypermetropia (+D), by setting the display frame as small as possible within the range of a predetermined standard (field angle) or other methods.